1. Field of the Invention
The invention relates to structures exhibiting three-dimensional periodicity, for example structures useful for photonic applications.
2. Discussion of the Related Art
Recently, there has been increasing interest in periodic dielectric structures, also referred to as photonic crystals (PC), in particular, photonic crystals exhibiting gaps in photonic band structures (referred to as photonic band gap (PBG) materials), for numerous photonic applications. See, e.g., P. S. J. Russell, xe2x80x9cPhotonic Band Gaps,xe2x80x9d Physics World, 37, August 1992; I. Amato, xe2x80x9cDesigning Crystals That Say No to Photons,xe2x80x9d Science, Vol. 255, 1512 (1993); and U.S. Pat. Nos. 5,600,483 and 5,172,267, the disclosures of which are hereby incorporated by reference. PBG materials exhibit a photonic band gap, analogous to a semiconductor""s electronic band gap, that suppress propagation of certain frequencies of light, thereby offering, for example, photon localization or inhibition of spontaneous emissions. A PC is generally formed by providing a high refractive index dielectric material with a three-dimensional lattice of cavities or voids having low refractive index. Photons entering the material concentrate either at the high-index regions or the low-index regions, depending on the particular energy of the photon, and the photonic band gap exists for photons of a particular energy between the two regions. Photons having energy within the PBG cannot propagate through the material, and their wave function thereby decays upon entering the material. The photonic band structure, therefore, depends on the precision of the physical structure and on its refractive index, and some difficulty has arisen in fabricating such materials. Specifically, it has been difficult to organize a three-dimensional lattice of micron scale, particularly with high refractive index materials.
In one approach, reflected in the above-cited U.S. Patents, solid materials are provided with numerous holes by mechanical techniques, e.g., drilling, or lithographic techniques, e.g., etching. This approach has provided useful results, but is limited by the ability of current processing technology to provide the necessary structure.
In another approach, ordered colloidal suspensions or sediments of relative low refractive index particles such as polystyrene, referred to as colloidal crystals, are used as templates for infiltration or deposition of high refractive index materials in a desired structure, and the particles are then etched away or burned out to provide the voids. See, e.g., B. T. Holland et al., xe2x80x9cSynthesis of Macroporous Minerals with Highly Ordered Three-Dimensional Arrays of Spheroidal Voids,xe2x80x9d Science, Vol. 281, 538 (July 1998); E. G. Judith et al., xe2x80x9cPreparation of Photonic Crystals Made of Air Spheres in Titania,xe2x80x9d Science, Vol. 281, 802 (July 1998); and A. A. Zakhidov et al., xe2x80x9cCarbon Structures with Three-Dimensional Periodicity at Optical Wavelengths,xe2x80x9d Science, Vol. 282, 897 (October 1998). The infiltration/deposition has been performed, for example, by an alkoxide sol-gel technique and by chemical vapor deposition. The results attained thus far have been interesting, but are far from providing a commercially feasible product. Specifically, the infiltration/deposition of the high refractive index material tends to be insufficient (e.g., low density leading to low refractive index) and non-uniform. For example, during alkoxide sol-gel deposition and CVD, some voids near the outside of the crystal become clogged, such that gelation/deposition at interior voids is inhibited. Moreover, insufficient and inadequate infiltration create voids within the high index material, causing substantial shrinkage, and thus cracking, during removal of the template material.
Thus, improved processes for fabrication of high density, substantially uniform photonic bandgap materials are desired.
The process of the invention provides a structure, e.g., a photonic band gap material, exhibiting substantial periodicity on a micron scale, the process providing improved density and mechanical integrity compared to current processes. (Periodicity, as used herein indicates that the structure is composed of a three-dimensional periodic array of repeated units. See, e.g., N. W. Ashcroft et al., Solid State Physics, 64, W. B. Saunders Co. (1976).) The process involves the steps of providing a template comprising a colloidal crystal, placing the template in an electrolytic solution, and electrochemically forming a lattice material, e.g., a high refractive index material, within the colloidal crystal. The colloidal crystal particles are then typically removed, e.g., by heating, etching, or dissolving, to form the desired structure. The electrode is generally oriented such that the electrodeposition substantially occurs along a plane moving in a single direction, in order to attain a desired density. Moreover, because the electrochemically grown lattice is itself a three-dimensionally interconnected solid, there is substantially no shrinkage upon subsequent treatment to remove the colloidal crystal. Useful lattice materials formed by electrochemistry include but are not limited to cadmium sulfide, cadmium selenide, zinc selenide, selenium, and a variety of metals.